Electronic control system for shaping the fuel-speed characteristics of an internal combustion engine

ABSTRACT

Additional amplifiers are connected in parallel to the high gain direct current amplifier of the parent application. These have a very high output impedance compared to the output impedance of the direct current amplifier. All amplifier outputs are connected to a voltage divider tap from which the fuel control signal is derived. The additional amplifiers have inherent temperature compensation and shape the full-load limiting line, while the original high gain direct current amplifier shapes the speed control lines.

United States Patent Schlimrne May 30, 1972 [54] ELECTRONIC CONTROL SYSTEM FOR 3,425,401 2/1969 Lang ..123/32 SHAPING THE FUELSPEED 3,063,242 11/1962 Bancroft. ...60/39.28 3,070,735 12/1962 Kaiser.... ...60/39.28 v CHARACTERISTICS OF AN INTERNAL 3,128,603 4/1964 Haigh ..60/39.28

COMBUSTION ENGINE FOREIGN PATENTS OR APPLICATION [72] Inventor: Ewald Schlimme, l-lildesheim, Germany I s 1,191,641 5/1970 Great Britain ..123/32 {731 Assgneei Smigmi Germany 1,125,718 3/1962 Germany ..123 32 [22] Filed: Oct. 30, 1969 Primary Examiner-Laurence M. Goodridge PP- 872,674 Assistant Examiner-Ronald B. Cox

Related Us. Application Dam Attorney-M1chael S. Striker [63] Continuation-impart of Ser. No. 867,025, Oct. 16, ABSTRACT Additional amplifiers are connected in parallel to the high gain direct current amplifier of the parent application. These [30] Forelgn Application Horny Data have a very high output impedance compared to the output Nov. 7, 1968 Germany ..P 18 07 539.3 impedance of h ir c rrent amplifier. A11 amplifier outputs are connected to a voltage divider tap from which the fuel 52 us. c1 ..123/32 EA, 123/139 E control signal is The additional amplifiers have 5 1 g, C 1102 3 00, 02, 39 00 herent temperature compensation and shape the full-load [58] Field of Search ..123 32 limiting line, while the Original high gain direct current p fier shapes the speed control lines.

[56] References Cited 17 Cl 13 Drawing g e UNITED STATES PATENTS 3,407,793 10/1968 Lang ..123/32 1 if? 1""? I I 24 4 1.1 56 21 i U U52 o A 2 .f I 28 ,U

i 31 l 1 (3G 3 Patented May 30, 1972 4 Sheets-5 1 FIG] his ATTORNEY Patented May 30, 1912 3,665,900

' 4 Sheets-Sheet 2 FIGS 38 27 nl g INVENTOR Ewe/d SCHL/MME 7 [US ATTORNEY Patented May 30, 1972 3,665,900

' 4 Sheets-Sheet I5 FIG. 7

1 I 02 ni n INVENTOR Ewu/d SCHL IMME By his ATTORNEY Patented May 30, 1972 4 Sheets Sheat 1 FIG. 70

UY T\ .4 nl n FIG. 77

IN VE N TOR Ewczld SCHL/MME hi5 ATTORNEY ELECTRONIC CONTROL SYSTEM FOR SHAPING THE FUEL-SPEED CHARACTERISTICS OF AN INTERNAL COMBUSTION ENGINE CROSS REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION ple, the point at which unburned fuel may be found in the exhaust of the motor) may require differentfull-load limiting lines, where the full-load limiting lines determine the maximum. amount of fuel to be injected as the load increases. Specifically, it may be necessary that the full-load limiting lines exhibit one or more breaks in their characteristics. I

SUMMARY OF THEINVENTION This invention comprises an electronic control system for an internal combustion'engine having at least one electrically controlled fuel injector means for injecting a quantity of fuel corresponding to a fuel control signal during each operating cycle. The system comprises means for furnishing a desired speed signal and means for furnishing an actual speed signal. It comprises high gain direct current amplifier means adapted to furnish said fuel control signal in response to said desired and actual speed signals. It further comprises additional amplifier means having a highoutput impedance relative to the output impedance of said high gain direct current amplifier means, connected in parallel to said high gain direct current amplifier means. The outputs of both the high gain direct current amplifier means and the additional amplifier means are connected to the voltage divider tap of 'a voltage divider connected from a DC voltage source to ground. The'output voltage, or fuel control'signal, is derived between ground and the voltage divider tap.

The novel features which are considered as characteristic for the invention are set forth in-particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I shows a number of different fuel-speed characteristic lines;

FIG. 2 is a basic block diagram of the control system for generating characteristic lines having a plurality of discontinuties or breaks; 1

FIG. 3 is a circuit diagram for generating a fuel-speed characteristic with positive slope;

FIG. 4 shows the characteristic lines corresponding to the circuit of FIG. 3;

FIG. 5 is a circuit diagram for generating a fuel-speed characteristic with increasing slope and having two discontinuties;

FIG. 6 shows the characteristic line generated by the circuit of FIG. 5;

FIG. 7 is a circuit diagram for an alternative embodiment for generating a fuel-speed characteristic of positive slope;

FIG. 8 shows the characteristic line generated by the circuit of FIG. 7;

FIG. 9 shows a circuit diagram for generating fuel-speed characteristic lines having negative slopes;

FIGS. 10 and 11 show the corresponding fuel-speed characteristics;

FIG. 12 is a circuit diagram of a further embodiment of a circuit for generating fuel-speed characteristics with negative slopes; and

FIG. 13 shows the characteristic lines'generated by the circuit of FIG. 12.

. DESCRIPTION or Tl-[E PREFERRED EMBODIMENTS FIG. 1 shows a number of possible fuel-speed characteristic lines. In these, the ordinate represents the quantity of fuel 0 injected during each operating cycle, while the abscissa represents the engine speed in revolutions per minute. In order to generate such or similar curves, the circuit of FIG. 2 is to be used. This shows a direct current amplifier 20 having a diode 21 connected to its output. This direct current amplifier has a feedback circuit'which, in the simplest case, is a resistor.

The high gain direct current amplifier has a first amplifier input labelled 22. An actual speed signal U, signifying the actual engine'speed is connected to terminal 22 via a resistance 24, while a signal generated by the movement of the accelerator pedal, also called a desired speed signal, is supplied to terminal 22 via a resistance 23. The other terminal of resistor 24, that is the terminal not connected'to terminal 22, is herein referred to as the input temiinal. The second amplifier input terminal, namely terminal 25 has a constant voltage U applied to it. Output circuit means, here a voltage divider comprising resistors 26 and 27 connected from the DC voltage source U to ground are connected to the anode of diode 21 whose cathode is connected to the output terminal of the high gain direct current amplifier. The common point of resistors 26 and 27 is herein referred to as the voltage divider tap. The fuelcontrol signal is derived from ground to the voltage divider tap. Additional amplifier means, labelled 28,29 and 30,

are connected in parallel with the high gain direct current amplifier circuit. The means for fumishing the actual speed signal are connected to the inputs of all of these amplifiers, each of which is designed to supply a predetermined part of the characteristic. Of course, further additionalamplifiers may be supplied in parallel with amplifiers 29 and 30 if further shaping of the fuel-speed characteristic lines is desired.

In order to move the horizontal portion of the characteristic lines, which horizontal portion is herein referred to as the fullload limiting line, in the direction of the ordinate independent of the voltage U current sources labelled 31 and 32 are provided which furnish current to the voltage divider tap. The currents furnished by these current sources may vary as a function of an operating parameter of the engine as, for example, the air pressure, the motor temperature, etc. Additional current sources may of course also be supplied if required.

It is the main point of this invention that a high gain direct current amplifier which furnishes the speed control lines, as the substantially vertical lines are herein referred to, has connected to it in parallel further amplifier means which have a high dynamic output impedance achieved by means of current feedback. These amplifiers each are designed to furnish a current relatively independent of load to the voltage divider tap to determine the top portion of the characteristic lines. In these type of circuits, the breaks or discontinuties in the fullload limiting line are generated at the input of the amplifier by use of the non-linear characteristic I. =1 se) or, ossibly, the non-linear characteristic of additional diodes. It should be noted that the high gain direct current amplifier which 3 voltage drop of the diode to be reduced by a factor coriesponding to the circuit amplification. However then it becomes necessary that the resistance of the feedback resistor is very high compared to the parallel resistance of the resistors 26 and 27 in order to prevent a direct interaction of U with the output voltage U,,. If this condition cannot be achieved, it is necessary to furnish an alternate feedback path comprising for example an Zener diode 57 in series with a diode 56, so that the feedback path is completed even when the diode is 21 is blocked. 7

Thus it is possible that the apparent short-circuit between inputs 22 and 25 remains in effect and the current flowing through the feedback resistance remains constant independent of U,,. Further, the arrangement of diode 21 shown in FIG. 2 has the advantage that the break between the full-load limiting line and the'speed control line is a sharp discontinuity. This is of course very desirable.

FIG. 3 shows a circuit for generating a full-load limiting line having a positive slope. It comprises a DC voltage source furnishing a voltage U It further comprises a transistor 33 having an emitter connected to an emitter-resistor 34 to which is supplied the actual speed signal, namely a voltage U,,. The

collector of transistor 33 is connected to the voltage divider tap. The circuit also comprises an additional voltage divider means connected in parallel to the voltage divider means comprising resistors 26 and 27. The additional voltage divider means comprise a resistance 37 in series with a resistance 38 connected from the DC voltage source to ground. The additional voltage divider means has an additional voltage divider tap which is the common point of resistors 37 and 38. To this is connected the cathode of a diode 36 whose anode is connected to the base of transistor 33. This diode is referred to as additional diode means. The anode of diode 36 is further con nected to the DC voltage source by a resistance 35.

FIG. 4 shows the characteristic lines associated with this circuit. The output voltage U, is plotted along the ordinate, while U,,, the voltage representing engine speed, is plotted along the abscissa. When U, is less than U,,,, a first predetermined speed, transistor 33 is conductive. The output current is then equal to:

I-Iere A is base circuit current amplification. The gain of this dU, R26 R27 dU. R26 1227 here U equals the forward voltage of the diode, U equal the base-emitter voltage, and S equal the gain of the transistor stage. Any temperature variation of U is compensated for by the diode voltage Up with sufficient accuracy.

The circuit of FIG. 3, can be further modified to generate a characteristic having two discontinuities, as shown in FIG. 5. This circuit comprises further voltage divider means, namely a resistor 39 connected in series with a resistor 40 from the DC voltage source to ground. The further voltage divider tap, namely the common point of resistors 39 and 40 is connected to the common point of emitter-resistor 34 and the cathode of an additional diode 41, whose anode has the actual speed signal of voltage supplied to it. In general, the circuit functions in the same manner as the circuit of FIG. 3. However, here, the emitter potential is fixed at the value U by means of the voltage divider comprising, resistors 39 and 40, thus generating a second discontinuity if U, is less than the value U (See FIG. 6). The portion of the curve with increasing slope is, however, temperature dependent because of the forward volt age U of diode 41. It is the function of diode 36 to render the portion of the characteristic line U less than U independent of temperature.

FIG. 7 shows a circuit which also generates characteristic lines of positive slope with two discontinuities, but has a better temperature stability than the circuit of FIG. 5. Here, the emitter-resistor 34 is connected to a voltage divider consisting of a transistor 42, or more specifically the emitter-collector circuit of transistor 42 connected in series with resistance means, here a resistor 40. The base of transistor 42 is connected to the tap of base circuit voltage divider means comprising a resistor 43 connecting the base to the DC voltage source and a resistor 44 connecting the base to ground. The collector of transistor 42 is connected to the DC voltage source, while the emitter is connected as previously stated to the emitter-resistor and also to the cathode of diode 41 whose anode receives the speed voltage 1],. Because of transistor 42, temperature compensation of the base-emitter voltage of transistor 33 may be achieved even for values of U,, less than U When U lies between the values of U,,, and U,,,, transistor 42 is blocked and the temperature compensation of U of transistor 33 is achieved by the forward diode voltage U If the base voltage divider means have a very low resistance (R43NR44 is much less than R40IIR34) then transistor 42 may be replaced by a diode 55, as indicated by dashed lines in FIG. 7. 1

FIG. 9 shows a circuit for'generating fuel-speed characteristic lines having a negative slope. This circuit is referred to as a second amplifier circuit. It comprises a transistor 45 whose collector is connected to the voltage divider tap furnishing the fuel control signal. lts'emitter is connected to ground via a first resistor, namely resistor 46. It is also connected to the DC voltage source by a second resistor, namely resistor 47. The base of the transistor is connected to the DC voltage source via'a third resistor, namely resistor 48, and is also connected to the anode of a diode 49 to whose cathode is furnished an actual speed signal. When U, is greaterthan U,,,, transistor 45 becomes conductive, thus generating the characteristic having a negative slope. The base-emitter voltage of transistor 45 is compensated by the temperature dependent forward voltage U of diode 49. In order to achieve the characteristic line shown in FIG. 11, the base of transistor 45 is connected to ground via a fourth resistor 50. This causes the base potential to be constant for speeds exceeding a fourth predetermined speed U Blocking of diode 49 for a voltage U, greater than U causes thesecond discontinuity.

Another embodiment similar to the circuit of 'FIG. 9, is shown in FIG. 12. Here a third transistor, namely transistor 51 has its emitter, referred to as a third emitter connected to the base of transistor 45, herein referred to as the second transistor. The third emitter, namely the emitter of transistor 51 is connected to ground, while the-third collector is connected to the DC voltage source by means of a resistance. The base of transistor 51, namely the third base, is connected to the DC voltage source by means of a sixth resistor, namely resistor 52 and is further connected to ground by means of a fifth resistor, namely resistor 53. The remainder of the circuit of FIG. 12 is identical to the circuit of FIG. 9. For a voltage U, greater than U the voltage U of transistor 45 is compensated for by the corresponding voltage of transistor 51. When U is less than U,,,, transistor 51 is blocked and the compensation is effected by the forward voltage U,, of diode 49.'If the base circuit voltage divider means have a low resistance, transistor 51 may be replaced by a diode, as shown with the dashed lines in FIG. 12 between the base of transistor 45 and the base of transistor 51.

Substantial advantages furnished by the embodiment described above, are the temperature stability of the amplifier circuit as well as the independence of the output current from the particular transistor used. The independenceof temperature is achieved by compensating the temperature dependency of the base-emitter voltage U of the actual amplifier transistor by the base-emitter voltage of another transistor, or the forward voltage U of a diode. The other advantage, namely the independence of theoutput current of the particular transistor used, is achieved by connecting the base of the amplifying transistor to a voltage source having a low internal impedance (U,. or voltage dividers) and feedback over a relatively large emitter-resistor. Thus the slope of the collector current (dIc)/(dU,,) is dependent only upon the current amplification A of the base circuit which is fairly independent of the particular transistor used.

While the invention has been illustrated and described as embodied in additional amplifier circuits using specific configurations, it is not intended to be limited to the'details shown, since various modifications and circuit changes may be made without departing in any way from the spirit of the present invention.

' What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. Electronic control system for internal combustion engines, comprising, in combination, at least one electrically controlled fuel injector means for injecting a quantity of fuel corresponding to a fuel control signal during each operating cycle; a source of DC. voltage; voltage divider means having a voltage divider tap, connected in parallel with said source of voltage; means for furnishing a desired speed signal signifying the desired engine speed; means for furnishing an actual speed signal signifying the actual engine speed; direct current feedback amplifier means connected to said means for furnishing a desired speed signal and said means for fumishing an actual speed signal, having a feedback amplifier output connected to said voltage divider tap and having a low dynamic output impedance; variable impedance means interconnected with said direct current feedback amplifier means for disconnecting said feedback amplifier output from said voltage divider tap under predetermined operating conditions; feedback means connected to said direct current feedback amplifier means for maintaining a predetermined voltage at said feedback amplifier output under said predetermined operating conditions; and additional amplifier means responsive to said actual speed signal, having an additional amplifier output connected. to said voltage divider tap, and having a high, dynamic output impedance relative to said low dynamic output impedance of said direct current feedback amplifier means over the whol actual speed signal range.

v 2. A control system as set forth in claim 1, further comprising a source of current connected in parallel to the parallel combination of said high gain direct current amplifier means and said additional amplifier means. v

3. A control system as set forth in claim 2, wherein the current furnished by said source of current varies as a function of an operating parameter of said engine.

. 4. A control system as set forth in claim I, wherein said additional amplifier means comprise an amplifying circuit adapted to furnish an additional output signal varying as a function of engine speed, in response to said actual speed signal, when said actual speed signal signifies an engine speed less than a first predetermined speed, and a constant additional output signal when said actual speed signal signifies an engine speed exceeding said first predetermined speed.

5. A control system as set forth in claim 4, wherein said amplifying circuit comprises a DC voltage source; a transistor having an emitter, collector and base; wherein said voltage divider means are connected across said DC voltage source; wherein the collector of said transistor is connected to said voltage divider tap; wherein said actual speed signal is supplied to the emitter of said transistor; further comprising additional voltage divider means connected in parallel to said voltage divider means and having an additional voltage divider tap; diode means having an anode connected to the base of said transistor and a cathode connected to said additional voltage divider tap; and resistance means interconnecting the collector of said transistor and said DC voltage source.

6. A control system as set forth in claim 5, further comprising means for furnishing a constant speed signal corresponding to a second predetermined engine speed to the emitter of said transistor until said engine speed exceeds said second predetermined engine speed.

7. A control system as set forth in claim 6, wherein said means for supplying a constant s d si al con-es ndin to a second predetermined engine sfid c i nprises fu riher vgltage divider means connected across said DC voltage source and having a further voltage divider tap; additional diode means interconnecting said additional voltage divider tap and said means for furnishing an actual speed signal; and an emitter-resistor having a first terminal connected to the emitter of said transistor and a second terminal constituting an additional amplifier input temrinal connected to said further voltage divider tap.

8. A control .system as set forth in claim 7, wherein said further voltage divider means comprise a first and second resistor. 1

9. A control system as set forth in claim 7, wherein said further voltage divider means comprise a voltage divider transistor having an emitter-collector circuit connected from said DC voltage source to said additional amplifier input terminal; further comprising base circuit voltage divider means having a base circuit voltage divider tap connected to the base of said voltage divider transistor.

10. A control system as set forth in claim 7, wherein said further voltage divider means comprise further diode means; and resistance means connected in series with said further diode means.

1 l. A control system as set forth in claim 1, wherein said additional amplifier means comprise a second amplifier circuit adapted to furnish a second additional output signal varying as a function of engine speed, in response to said actual speed signal, when said actual speed signal signifies an engine speed exceeding a third predetermined speed, and adapted to furnish a second constant output signal when said actual speed signal signifies a speed less than said third predetermined speed. i

12. A control system as set forth in claim 11, wherein said second amplifier circuit furnishes an additional output signal decreasing with increasing engine speed when said actual speed signal signifies an engine speed greater than said third predetermined speed.

13. A control system as set forth in claim 12, further comprising means for maintaining said second additional output signal constant when said engine speed exceeds a fourth determined speed. v v 1 14. A control system as set forth in claim 12, wherein said second amplifier circuit comprises a second transistor having 'a second emitter, a second collector, and a second base, said collector being directly connected to said voltage divider tap; temperature compensating diode means interconnecting said means for furnishing an actual speed signal and the base of said transistor; a first resistor connecting said emitter to ground; a DC voltage source; a second resistor connecting said emitter to said DC voltage source; and a third resistor connecting said base to said DC voltage source.

15. A control system as set forth in claim 14, further comprising a fourth resistor connecting said base to ground.

16. A control system as set forth in claim 14, further comprising a third transistor having a third base, a third emitter, and a third collector, said third emitter being connected to said second base, said third collector being connected to ground; fifth resistor connecting said third base to ground; and a sixth resistor connecting said third base to said DC voltage source.

17. A control system as set forth in claim 15, further comprising second temperature compensating diode means, said second temperature compensating diode means having an anode connected to said second base and a cathode connected to said fourth resistor.

v l k 

1. Electronic control system for internal combustion engines, comprising, in combination, at least one electrically controlled fuel injector means for injecting a quantity of fuel corresponding to a fuel control signal during each operating cycle; a source of D.C. voltage; voltage divider means having a voltage divider tap, connected in parallel with said source of voltage; means for furnishing a desired speed signal signifying the desired engine speed; means for furnishing an actual speed signal signifying the actual engine speed; direct current feedback amplifier means connected to said means for furnishing a desired speed signal and said means for furnishing an actual speed signal, having a feedback amplifier output connected to said voltage divider tap and having a low dynamic output impedance; variable impedance means interconnected with said direct current feedback amplifier means for disconnecting said feedback amplifier output from said voltage divider tap under predetermined operating conditions; feedback means connected to said direct current feedback amplifier means for maintaining a predetermined voltage at said feedback amplifier output under said predetermined operating conditions; and additional amplifier means responsive to said actual speed signal, having aN additional amplifier output connected to said voltage divider tap, and having a high dynamic output impedance relative to said low dynamic output impedance of said direct current feedback amplifier means over the whole actual speed signal range.
 2. A control system as set forth in claim 1, further comprising a source of current connected in parallel to the parallel combination of said high gain direct current amplifier means and said additional amplifier means.
 3. A control system as set forth in claim 2, wherein the current furnished by said source of current varies as a function of an operating parameter of said engine.
 4. A control system as set forth in claim 1, wherein said additional amplifier means comprise an amplifying circuit adapted to furnish an additional output signal varying as a function of engine speed, in response to said actual speed signal, when said actual speed signal signifies an engine speed less than a first predetermined speed, and a constant additional output signal when said actual speed signal signifies an engine speed exceeding said first predetermined speed.
 5. A control system as set forth in claim 4, wherein said amplifying circuit comprises a DC voltage source; a transistor having an emitter, collector and base; wherein said voltage divider means are connected across said DC voltage source; wherein the collector of said transistor is connected to said voltage divider tap; wherein said actual speed signal is supplied to the emitter of said transistor; further comprising additional voltage divider means connected in parallel to said voltage divider means and having an additional voltage divider tap; diode means having an anode connected to the base of said transistor and a cathode connected to said additional voltage divider tap; and resistance means interconnecting the collector of said transistor and said DC voltage source.
 6. A control system as set forth in claim 5, further comprising means for furnishing a constant speed signal corresponding to a second predetermined engine speed to the emitter of said transistor until said engine speed exceeds said second predetermined engine speed.
 7. A control system as set forth in claim 6, wherein said means for supplying a constant speed signal corresponding to a second predetermined engine speed comprises further voltage divider means connected across said DC voltage source and having a further voltage divider tap; additional diode means interconnecting said additional voltage divider tap and said means for furnishing an actual speed signal; and an emitter-resistor having a first terminal connected to the emitter of said transistor and a second terminal constituting an additional amplifier input terminal connected to said further voltage divider tap.
 8. A control system as set forth in claim 7, wherein said further voltage divider means comprise a first and second resistor.
 9. A control system as set forth in claim 7, wherein said further voltage divider means comprise a voltage divider transistor having an emitter-collector circuit connected from said DC voltage source to said additional amplifier input terminal; further comprising base circuit voltage divider means having a base circuit voltage divider tap connected to the base of said voltage divider transistor.
 10. A control system as set forth in claim 7, wherein said further voltage divider means comprise further diode means; and resistance means connected in series with said further diode means.
 11. A control system as set forth in claim 1, wherein said additional amplifier means comprise a second amplifier circuit adapted to furnish a second additional output signal varying as a function of engine speed, in response to said actual speed signal, when said actual speed signal signifies an engine speed exceeding a third predetermined speed, and adapted to furnish a second constant output signal when said actual speed signal signifies a speed less than said third predetermined speed.
 12. A control system as set forth in claim 11, wherein said second amplifier circuit furnishes an additional output signal decreasing with increasing engine speed when said actual speed signal signifies an engine speed greater than said third predetermined speed.
 13. A control system as set forth in claim 12, further comprising means for maintaining said second additional output signal constant when said engine speed exceeds a fourth determined speed.
 14. A control system as set forth in claim 12, wherein said second amplifier circuit comprises a second transistor having a second emitter, a second collector, and a second base, said collector being directly connected to said voltage divider tap; temperature compensating diode means interconnecting said means for furnishing an actual speed signal and the base of said transistor; a first resistor connecting said emitter to ground; a DC voltage source; a second resistor connecting said emitter to said DC voltage source; and a third resistor connecting said base to said DC voltage source.
 15. A control system as set forth in claim 14, further comprising a fourth resistor connecting said base to ground.
 16. A control system as set forth in claim 14, further comprising a third transistor having a third base, a third emitter, and a third collector, said third emitter being connected to said second base, said third collector being connected to ground; fifth resistor connecting said third base to ground; and a sixth resistor connecting said third base to said DC voltage source.
 17. A control system as set forth in claim 15, further comprising second temperature compensating diode means, said second temperature compensating diode means having an anode connected to said second base and a cathode connected to said fourth resistor. 